OpenSSL 1.1.0 supports the use of this environment variable for
passing to the build files.  For the sake of backward compatibility,
we keep it.
Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NBernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/6668)